For example, in a projector for an optical device used for image display like a liquid crystal projector or a DLP (Trademark) projector, a high intensity discharge lamp (HID lamp) is used. In such a projector, by a dichroic prism etc., light is separated into the three primary colors of red, green, and blue, i.e., a space modulation element provided for each color generates an image of each of the three primary colors, and optical paths thereof are combined by a dichroic prism etc., so as to display a color image.
In another known type of projector, light emitted from a light source is passed through a rotating filter (dynamic color filter) comprising a transmission color wheel having three primary color areas (R, G, and B), thereby sequentially generating light rays of the three primary colors. In synchronization with the generated light rays, the spatial modulation device is controlled so as to sequentially generate an image of each of the three primary colors in a time dividing manner, thereby displaying a color image. When brightness is important, W, that is, white may be added to the three primary colors of R, G and B, so that the dynamic color filter has four colors, whereby an color image may be displayed by sequentially generating these colors, one by one. Or, furthermore, a color image may be displayed, by providing much more color areas thereon, thereby reinforcing color expression capability.
In a discharge lamp lighting apparatus for lighting a discharge lamp as described above, while voltage called a no-load open circuit voltage is impressed to a lamp at start-up, high voltage is impressed to the lamp, in order to generate dielectric breakdown in an electrical discharge space, so that the discharge state changes from glow discharge to arc discharge, and finally, the discharge lamp lighting apparatus is operated so that stable regular lighting may be realized. Usually, such a discharge lamp lighting apparatus has a converter which adjusts an output of an input power supply to a lamp discharge voltage, in order to realize a predetermined input power supplied to the lamp thereby outputting the required lamp current. Moreover, lamp voltage, i.e., output voltage of the converter, is detected, and based on this information, target lamp current is determined according to a value of the quotient which is obtained by dividing, for example, the target electric power by the detection voltage.
Although the lamp discharge voltage in a lighting steady state, i.e., lamp voltage, (VL) becomes low as the distance between tips of two electric discharge electrodes, i.e., the distance between electrodes, becomes short. However, since the light source becomes close to a point light source as the distance between electrodes becomes short, the use efficiency of light emitted from a lamp becomes high. On the other hand, when the same electric power is supplied to the lamp, because of decrease of the lamp voltage (VL), since lamp current (IL) becomes large, there is a disadvantage that generation of heat of a discharge lamp lighting apparatus increases. On the contrary, the light use efficiency becomes low due to a decrease (or losing) of the nature of a point light source as the distance between electrodes becomes long. On the other hand, when the same electric power is supplied to the lamp, because of an increase of the lamp voltage (VL), since only small lamp current (IL) is required, there is an advantage that generation of heat in a discharge lamp lighting apparatus can be reduced.
Therefore, it is not necessarily so advantageous if the distance between electrodes long or it is not necessarily advantageous if it is short. That is, it can be understood that the distance between the electrodes is required to be maintained in a desired range between the maximum and minimum which are determined from the brightness required for a light source of a projector and the limit of heat generation at which the discharge lamp lighting apparatus can be operated.
As types of discharge lamp driving methods, there are a direct current driving method in which a lamp is turned on by a converter, and an alternating current driving method in which periodic polarity reversals are performed by additionally providing an inverter further in the downstream side of the converter. In the case of the direct current driving method, since the light flux from the lamp is like direct current, that is, it does not change with passage of time, basically, there is a big advantage that it can be similarly applied to both types of the above-described projectors.
On the other hand, while in the case of the alternating current driving method, there are disadvantages, resulting from existence of polarity reversals, that is, overshoot, instantaneous light-out at the time of polarity reversals etc. occur, thereby causing a bad influence on a display image etc, there is an advantage that growth or consumption of electrodes of the discharge lamp may be controlled, by using the flexibility of polarity-reversal frequency, which the direct current driving method does not have.
In Japanese Laid Open Patent No. 2001-312997, the conventional technology is described. That is, in order to maintain the distance between electrodes in the desired range, growth or consumption of the electrode of a discharge lamp is controlled by controlling polarity-reversal frequency etc. Specifically, the laid open patent discloses that when a value of the distance between electrodes decreases from a regular value due to formation of a projection portion in the portion which faces the electrode tip section of a high-pressure discharge lamp, the frequency is set to a first frequency. When the projection portion becomes small, and the distance between electrodes increases from a regular value, the frequency is set to a second frequency.
Moreover, for example, Japanese Laid Open Patent No. 2002-175890 discloses a technology in which in a lamp driven by an alternating current and including electrodes having regulation withstand current, a period during which the frequency is 5 Hz or less is one (1) second or more, or a period during which lighting current becomes a rated current value or more is one (1) second or more, is provided.
Furthermore, for example, Japanese Laid Open Patent No. 2003-133091 discloses a technology in that when the voltage between electrodes falls below a predetermined value due to a change of the distance between electrodes during lighting, a period during which an alternating current at a frequency lower than a rated frequency is applied is temporarily provided.
Furthermore, for example, Japanese Laid Open Patent No. 2003-338394 discloses a technology in that in case a lamp is turned on with electric power lower than rated power, when the voltage between electrodes falls below a predetermined value due to a change of the distance between electrodes, a period during which an alternating current at a frequency higher than a frequency of lighting current is provided for only a predetermined time.
Furthermore, for example, Japanese Laid Open Patent No. 2004-342465, discloses a technology in that for a certain period of time immediately after starting an electric-discharge lamp, a polarity-reversal operation of a full bridge circuit is carried out at an alternation frequency which is easy to form a projection of an electrode, and after the certain period of time, a polarity-reversal operation is carried out at an alternation frequency which causes little change to the electrodes. Moreover, the Japanese Laid Open Patent discloses that the alternation frequency is increased as the tube voltage of an electric-discharge lamp rises, and further the time ratio of positive polarity and negative polarity in the polarity reversal operation is changed depending on the state.
Furthermore, for example, Japanese Laid Open Patent No. 2005-197181 discloses a technology in that polarity-reversal frequency is changed to two or more levels, according to the magnitude relation between a lamp voltage and a switching voltage, and a lamp is turned on at a predetermined fixed frequency for a predetermined period after start-up.
Furthermore, for example, Japanese Laid Open Patent No. 2006-140016 discloses a technology in that the frequency of alternating current is regularly or irregularly changed.
Furthermore, for example, Japanese Laid Open Patent No. 2006-156414 discloses a technology in which a bridge drive frequency is controlled to be switched between two or more frequencies at the time of lighting.
Furthermore, for example, Japanese Laid Open Patent No. 2006-185663 discloses a technology in which a polarity-reversal frequency of a bridge is changed according to a lamp voltage.
Furthermore, for example, Japanese Laid Open Patent No. 2007-087637 discloses that when a lighting voltage of a discharge lamp is a first predetermined value or more, the lamp is turned on at a low frequency, and when the lighting voltage of the discharge lamp is a second predetermined value or less, the lamp is turned on without inserting the low frequency.
However, in such conventional technologies, sufficient controllability to maintain the distance between electrodes in the desired range could not be realized. Uncontrollability of the distance between electrodes is remarkable in a lighting condition in which electric power smaller than the actual lamp power capacity is applied to the lamp by lighting modulation. This is because the electrode temperature of the lamp tends to become low on such low electric power conditions, so that a projection at the tip of the electrode tends to grow, and in a response to the lamp voltage drop with the growth of a projection, a lamp current is increased in order to realize a target electric power, so that the efficiency of the weak feedback loop is not good until the temperature of the electrode comes to a rise. Therefore, when the lamp voltage begins to decrease after exceeding the desired range, there is a problem that it takes time to return to the desired range since the lamp voltage cannot be immediately changed to a rise, depending on the conventional technology.